Electrical control system



H. c. GRANT, JR., ETAL 3,234,536

ELECTRICAL CONTROL SYSTEM 4 Sheets-Sheet 1 Feb. 8, 1966 Filed Feb. 20, 1963 l Feb. 8, 1966 H. c. GRANT. JR., ETAL 3,234,536

ELECTRICAL CONTROL SYSTEM Filed Feb. 20, 1963 4 Sheets-Sheet 2 HARRY C-GRANTy JR- BYMCOLM W. LOIHLE a W7 G E NT G5 INVENTORS Feb. 8, 1966 H. C GRANT, JR, ETAL 3,234,536

ELECTRICAL CONTROL SYSTEM 4 Sheets-Sheet 5 Filed Feb. 20, 1963 INVENTORS HARRY C GRANT. JR. MALCOLM W- LOIH LE M' AGENT Feb 8, 1966 H. c. GRANT. JR., ETAL 3,234,535

ELECTRICAL CNTROL SYSTEM 4 Sheets-Sheet 4 Filed Feb. 20, 1965 OM T lll l Il. 09+ 5; E: www@ All OwHdJDwmZ Om+ A All. lvU/Al United States Patent C) 3,234,536 ELECTRICAL CONTROL SYSTEM Harry C. Grant, Jr., Ridgewood, and Malcoim W. Loihle,

Florham Park, NJ., assignors to Specialties Development Corporation, Belleville, NJ., a corporation of New `lersey Filed Feb. 20, 1963, Ser. No. 260,007 Claims. (Cl. 340-214) The present invention relates to electrical control systems, and, more particularly, to such systems for controlling smoke detecting apparatus which automatically monitors a plurality of spaces in which smoke is to be detected.

The present invention, although useful for other purposes, is primarily concerned with an electrical control system for automatically operating, supervising and testing the smoke detecting apparatus, shown and described in co-pending application for Letters Patent Serial No. 229,807, tiled October ll, 1962.

ln smoke detecting apparatus ,of the type disclosed in the above identified co-pending application, a plurality of smoke conducting conduits extending from the spaces being monitored are connected through a selector valve device to a smoke detecting unit. The control system for such apparatus normally operates the selector valve device to sequentially connecteach of the conduits to the detecting unit, and interrupts the operation of the valve device when the detecting unit responds to the presence of smoke. In previously known control systems, the smoke detecting unit could become inoperative or the selector valve device could cease to operate and such failures could be detected only by close scrutiny of the apparatus, if at all.

In smoke detecting apparatus of this type, a photoelectric cell is normally provided in a chamber where it is affected by the presence of smoke to produce a signal. The chamber is connected to the selector valve by a suitable length of pipe through which smoke laden air must move before it reaches the detector. Therefore, should smoke initially enter this connecting pipe just before the selector valve changes the line connected to Ithe detector, an erroneous indication that smoke is present in the space associated with the subsequently connected line can result.

Accordingly, an object of the present invention is to provide an improved electrical .control system for smoke detecting apparatus.

Another object is to provide such a system for automatically operating, and monitoring and testing the condition of the smoke detecting apparatus.

Another object is to provide such a system including an arrangement for preventing the smoke detecting apparatus from giving an erroneous indication .as to the source of detected smoke.

Another object is to provide such a system including an arrangement for automatically and periodically testing the operativeness of the smoke detecting apparatus.

Another object is to provide such a system including an arrangement for monitoring the operation of the selector valve and automatically giving a false indication should the selector valve cease operating.

Another object is to provide such a system including an arrangement for stopping the selector valve and inerting the monitoring arrangement when smoke is detected.

Another object is to provide such a system wherein the operation of the detecting apparatus is tested once during each cycle of the selector valve without activating the arrangement for stopping the selector valve.

Another object is to provide such a system including improved means for operating the selector valve.

A further object is to provide such a system which is reliable, accurate, has long life, and requires a minimum of maintenance.

ICC

Other and further objects of the invention will be obvious upon an understanding of the illustrative embodiment about to be described, or will be indicated in the appended claims, and various advantages not referred to herein will occur to one skilled in the art upon employment of the invention in practice.

A preferred embodiment of the invention has been chosen for purposes of illustration and description, and is shown in the accompanying drawings, forming a part of the specification, wherein:

FIG. 1 is a schematic block diagram of an electrical control system according to the present invention.

FIGS. 2, 3 and 4 are Wiring diagrams which when arranged with FIG. 3 below FIG. 2 to align lines .l to P and FIG. 4 to the right of FIG. 2 to align lines A to H provide an entire wiring diagram for the system shown in FIG. 1.

Referring to the drawings in detail and particularly to FIG. l thereof, there is shown a smoke detecting system including smoke detecting apparatus and an electrical system in accordance with the present invention for controlling the smoke detecting apparatus. The smoke detecting apparatus, which is disclosed in detail in the aforementioned co-pending application, generally com* prises `a photo-electric smoke detecting unit lil, an eight position rotary selector valve 11 having an outlet conduit 12 connected to `the air intake of the detecting unit 10, seven input conduits 13 extending from the selector valve 11 to a plurality of spaces which are to be monitored by the detecting apparatus, an air blower 14 connected to the air outlet `of the detecting unit 10, and a motor arrangement for driving the selector valve including a continuously acting torque motor 15 and a stepping brake 16 mounted on a common shaft 17 with the selector valve 11. A number wheel 1 8 is also provided on the shaft 17 to indicate which conduit 1.3 is connected to the detecting unit 10.

The stepping brake 16 includes a disc 19 secured to the shaft 1 7 for rotation therewith, four armatures 20 `on the dise 19 equally spaced about the circumference thereof, and two pairs of diametrically opposite electromagnets 21 and 22 positioned adjacent to the disc 19 to cooperate with the armatures 20 to provide a braking force on the shaft 17. The pair of magnets 21 are positioned forty-five degrees in advance of the pair of magnets 22.

The torque motor 15 constantly urges the shaft 17 to rotate, and ,the stepping brake 1.6 Periodically interrupts the rotation of the shaft 17 so that the selector valve 11 moves in eight steps to connect each of the conduits 13 to the smoke detecting unit lil for a predetermined period of time during each revolution of the valve. The blower 14 draws air from the spaces to be monitored through the conduits 13 and the selector valve 11 into the smoke detecting unit 1i) Which produces an electrical signal if smoke is present.

GENERAL DESCRIPTlON OF CONTROL SYSTEM The electrical control system for the apparatus just described generally comprises a timing pulse generator 24, a brake driver 25 controlled by the pulse generator for alternately energizing the electro-magnets 21 and 22 to operate the stepping brake 16, an amplifier 26 connected to the output of the detecting unit 10, an alarm energizing circuit 27 connected .to the output of the amplifier 26, an alarm unit 29 under the control of the energizing circuit 27 for giving an indication when the detecting unit 1l) responds to the presence of smoke, a pulse generator control circuit 30 for interrupting the operation of the pulse generator to stop the selector valve when an alarm is given, an alarm hold-off circuit 31 to insure that the valve 11 stops only on a conduit 13 delivering smoke, a motion supervising arrangement for giving an indication should ,the selector valve fail to rotate, and a testing arrangement for testing the operativeness of the smoke detecting unit.V

The alarm .hold-off circuit 31 comprises a switching circuit 32, a delayed timing pulse generator 34, and a smoke detector shunting circuit 35. K

The motion supervising arrangement comprises a motion sensing unit 36 adjacent the disc 19, a pulse former 37 connected to an output of the brake driver 25, a motion supervisor circuit 39 responsive to the outputs of the motion sensor 36 and the pulse generator 37, a fault indicator 40 controlled by the circuit 39, and a fault signal preventing circuit 41 connected between the alarm unit 29 and the supervisor circuit 39.

The testing arrangement comprises an automatic test switch 42 positioned adjacent the disc 19, a test switch operator 44 mounte-d on the disc 19, a manual test switch 45, a smoke simulator 46 in the detector unit 16 under control of the test switches 42 and 45, an alarm hold-on circuit 47 under the control of the automatic test switch and the alarm hold-off circuit 31, andI an integrity supervisor circuit 4S, The detecting unit 10 and the smoke simulator 46 may be of the type disclosed in co-pending application Serial No. 212,097, led July 24, 1962.

To make the information provided by the system available at another location, a repeater R is connected to the alarm unit 29, the fault indicator 4t), and the outputs of the brake driver 25 to providel duplicate alarm, fault and selector valve position indications. The duplicate alarm and fault indications are supplied by providing appropriate indicating equipment electrically connected with the equipment provided in the alarm unit 29 and the fault indicator 40. The duplicate selector valve position indication is provided by a torque motor, indexing means, and a number wheel arrangement. Such a repeater is disclosed in co-pending application Serial No. 265,740, tiled March 18, 1963.

GENERAL OPERATION OF THE CONTROL SYSTEM The timing pulse generator 24 delivers a positive output pulse of short duration every five seconds to the brake driver 25. The brake driver 25 has twooutputs, and, as described hereinafter in detail, is constructed so that one or the other of these outputs is always energized.

v Each output pulse of the pulse generator acts on the brake driver to reverse the condition of the outputs, that is, the previously energized loutputs is de-energized and the previously de-energized output is energized. The output signal of each of the driver outputs is therefore a series of squareV wave pulses of live second duration separated from each other by iive second intervals, the pulses in Vone output occurring during the interval between the pulses in the other output.

-The brake driver, therefore, energizes the electromagnets 21 for live seconds, then energizes the electromagnets 22 for five seconds and so forth. With referenceV to the waveforms shown in FIG. 1 for the output of the'driver, during the interval To to T5 the electromagnets 21 are energized and co-act with a pair of the armatures 20 to hold the disc 19 stationary against the action of the torque motor 15. At the time T5 the electromagnets 21 are deenergized and the electromagnets 22 are energized. Since there are no armatures 20 adjacent the magnets 22 at this time, the motor 15 drives the disc 19 to move a pair of armatures 20 into alignment with the energized magnets 22. The disc 19 is then held in this position until the time T10.

Each time the disc 19 is stopped, the selector valve 11 stops in a position connecting a different one of the conduits 13 through the conduit 12 to the smoke detecting unit 10. When smoke is present in the line 13 connected to the conduit 12, the unit 10 produces an output which is amplified by the amplier 2.6 and actuates the alarm energizing circuit 27 to deliver a signal to the alarm unit 29. The output signal of the circuit 27 is also transmitted to the pulse generator control circuit 30, which, in response to this signal interrupts the operation of the pulse generator 24. Since the pulses to the brake driver 25 are interrupted, the output of the driver energized at that time remains energized to hold the selector valve stationary s'o that the smoke filled line 13 remains connected to' the detecting unit 10. The position of the number wheel 18 indicates the source of thevsmoke. The selector valve 11 may be stopped at any time by operation of a manual stop switch 49.

When desired, for example after thersource of smoke has been determined, the system is reset in a manner described hereinafter and the pulse generator 24 is placed in operation again.

A finite period of time is required to move air through the conduit 12 from the valve 11 to the detector 10. Therefore, if smoke was introduced into the conduit 12 just prior to the time for movement ofthe valve 11 to its next position, the smoke would not reach the detecting unit until after the valve had moved to its next position. The detector therefore could give an alarm when the selector valve is actually connected to a smoke-free conduit following a smoke-bearing conduit. Such an alarm would stop the operation of the selector valve 11 and give an erroneous indication as to the location of the smoke source.

To prevent such erroneous indications, the alarm holdoff circuit 31 serves to render the detecting unit 10 ineffective during the first three seconds the selector is in a given position to allow the conduit 12 and the detecting unit 10 to be cleared of any smoke introduced from the preceding conduit. To provide this three second holdoff, the output of the timing pulse generator 24 is applied to the switching circuit 32, The switching circuit 32 normally applies a direct current signal to the shunting circuit 35. Each pulse produced by the timing pulse generator 24 actuates the switching circuit 32 to interrupt this direct current signal 4and simultaneously activates the delayed pulse generator 34. After a three second delay, the delayed pulse generator 34 applies a pulse to the switching circuit`32 to re-establish the direct current signal to the shunting circuit 35. The switching circuit 32 thus responds to the outputs of the delayed pulse generator 34 and the pulse generator 24, and delivers a series of pulses of two second duration to the shunting circuit 35.

The shunting circuit 35 normally prevents any output of the detecting unit 10 from being applied to the ampliiier'26 and is rendered ineffective by the output of the' switching circuit 32 during the last two seconds of the vselector Valve 11 is in each position. VFor example, at the time T5 when the selector. valve Y11 moves to a new position, thesoutput of the switching circuit 32 disappears. The shunting circuit" 35 short circuits any output of the detecting unit 10 to prevent the ampliiier 26 from receiving an input. Three seconds later the time T8 the switching circuit 32 responds tothe output of the delayed pulse generator 34 and produces an output which renders the shunting circuit 35 inoperative. From the time T8 to the time T10' anyy output of the detecting unit 10 is applied to the amplifier 26. At the time T10 the swit-ching ycircuit 32 `responds to the output pulse of the pulse generator 24 and the signal applied to the shunting circuit disappears as the selector valve moves to its next position.

Each time the electromagnets 22 are energized during the rotation of the selector valve, the armature 20 moves into alignment with one of the electromagnets 22, passes the motion sensing unit 36 and causes this unit 36 tof produce an output pulse. An larmature 20 passes the motion sensing unit 36 lonce in every two steps, there-- fore, the output pulses of the unit are ten seconds apart.. The output of the brake driver 25 for energizing the electromagnets 21 is passed through the pulse former 37` which transforms the leading edge of each square wave into `a lshaxp pulse. These pulses are also ten seconds apart and occur in time between the output pulses ofV the 'motion sensing unit 36. The motion supervisor circuit 39 therefore receives an input pulse every live seconds.

If rnotion ot `the disc 19 should stop for 4any reason, the motion sensing unit 35 produces no output a-nd the motion supervisor circuit 39 then receives pulses only every l0 seconds from the pulse former 37. The motion supervisor circuit 39 is constructed and arranged, as described hereinafter in detail, to operate the fault indicator 40 in the event that no .pulse is received lfor a predeter-mined period of time more than tive seconds but less than ten seconds, for example, eight seconds.

When the alarm unit 29 Yis actuated in response `to the detection of smoke, the fault signal preventing circuit 41 is actuated and .produces an output signal which inerts the motion supenvisor circuit 39 so that a fault indication is not ygiven as a result of the stopping off the disc 19 through the operation of lthe control circuit 30.

During each revolution when the selector valve 11 moves through its eighth or test position wherein it is not connected to any of the seven Vconduits 14, lthe test switch operator '44 contacts yand actuates the switch 42 to energize the smoke simulator i6k and send signals `to both .the control circuit 30 and the alarm hold-on circuit 47.

The smoke simulator 46 affects the photocell of the detecting unit in a marmer whereby, if the detecting unit 10 is in proper operating condition, `an output signal is produced which is suiicient to cause the operation of the alarm 29. However, the hold-oit circuit y31 V.ternporarily prevents this signal from operating the alarm 29. The hold-on circuit 47 responds to the signal from the test switch 42 and immediately energizes the alarm unit 29. The alarm unit 29, however, is Iprevented `from giving `an indication by other testing circuitry described in detail hereinafter. The alarm hold-on circuit 47 remains energized for the irst lthree seconds that the selector valve 11 is in this position. At the end ofl the three second interval, the output of the switching circuit 32 appears and de-energizes the hold-on circuit, and the holdoft circuit .is turned off to allow the output of the unit 10 to be ted through the ampliiier 26.

It the detecting unit 10, the amplifier 26, the alarm energizing circuit 27, or the alarm uni-t 29 is not operating properly, the integrity supervisor circuit 43 operates at this time to give a fault indication during. the inal two seconds that the selector valve 11 is in this position.

ri`he control circuit 30, in response to the signal Ifrom the test switch 42, is rendered insensitive to the stop motion signal transmitted thereto when the alarm energizing `circuit 27 .is activated by the operation `of the holdon circuit d'7. Therefore, the timing pulse generator 24 continues to operate and the selector valve is moved without interruption to its next position at the end of the five `second interval.

In addition to the above described automatic testing, the system may be tested at any time and in any position by operation of the manual test switch 45.

DETAILED DESCRPTION OF THE LCONTROL SYSTEM Referring now to the detailed wiring diagram shown in FiGS. 2 to 4, the system is powered by a constant voltage transformer 50 providing yconstant 115 volt and 7 volt alternating current power, and by a regulated rectiiier 51 providing 30 volt and 100 volt direct current power.

'il-he timing pulse generator 24 (FIG. 2) includes a unijunction transistor 52 under the control of a RC timing network. The unijunction transistor S2 has a loase 1 electrode 53 connected through a 330 ohm resistor 54`to a posi-tive conductor S5, a base 2 electrode 56 connected through a 33 ohm resistor 57 to a negative conductor 59, and an emitter electrode 60 connected to the RC timing network. The timing network includes a 500,000 ohm variable resistor 61, a 250,000 ohm Variable resistor 62, -and a 100,000 ohm lresistor 64 connected inv 4series between the .conductor 55 and 4the emitter 60; an 8 microiarad capacitor 65 connected between the emitter 60 and the conductor 59; and a 6 microtarad capacitor '66 connected in parallel -with the resistors 61, 62, and 64, between the emitter 60 and the conductor S5. A fast step switch-67 is connected across the resistor -6-1 and the output of the timing pulse generator is taken across the resistor 57.

The bra-ke driver 25 (LFIG. 2) includes a controlled silicon rectifier T69 connected in series with 'thel electromagnets 22 between the conductor 55 and the conductor 59, -and al second controlled silicon rectifier 70 connected `in series with the electromagnets 2.1 between the conductors 55 and 59. 'Phe `output signal 'of the pulse generator IZei is coupled -to the brake driver 25 by 4means of a .047 microfarad capacitor 71 anda 680 oh-m resistor 72. This signal is applied to gate electrodes 74 and 75 of the controlled rectiers 69 `and 70 through diodes 76 and 77 respectively.

To complete the incoming signal circuits, the `gate electrodes 74 land 75 are respectively connected through 2200 ohm resistors 79 and 80 to the points of ljunction X and Y of the controlled rectifiers 69, 70 with the brakes 22, 21 respectively. VA pair of 18-.rnicrofarad capacitors 811, 82 are connected in parallel between these junction points.

The brake `driver is lprovided with a starting circuit including a transistor 84, a .15 0 ohm resistor 85 connecting the collector of the transistor to the conductor 55, a 10,000 ohm resistor 86 connecting Ythe base of the transistor to the conductor 59, a 360 ohm resistor 37 connected between the emitter and the collector of the transistor, a diode 89 and a 150 ohm resistor l90 connected fin series between the collector of the transistor and the gate electrode 75 of the controlled rectier 70, a 1,000 ohm resistor 91 `having one end connected to the base of the transistor, and a pair of diodes 92, 94 respectively connected between the points X and Y and .the other end of the resistor 91.

The smoke detecting unit 10 (FIG. 3) .includes a lamp 95 and a photoelectric cell 96 connected through a 10,000 ohm resistor 97 to the input of the amplifier 26.

The ampliiier26 includes three tnansistors 99, 100, and 101 connected in a cascade emitter follower arrangement, 'a variable gain adjust resistor 102, and a voltage divider arrangement comprising a resistor 104 and a resistor 10S connected in series between a positive yconductor 106 anda negative conductor 107. The collectors of each of the transistors 99, 100, 101are connected to the junction of resistors 104 and 105, and the emitters thereof are respectively connected through resistors 109, 110 and 111to the negative conductor 107. A capacitor 112 is connected in parallel with the resistor 109. The input resistor l102 is connected between the base of the transistor 99 and the nega-tive conductor 107, `and the output of the amplifier is taken across the resistor 111.

The `alarm energizing circuit 27 includes a controlled silicon vrectifier 113 connectedy in series with a reset-switch 114 and the coil 115 of relay 116 between the output of 'the switching circuitl 32 and 'the negative conductor 107. The outputy of the amplifier 26 is connected to the gate 4electrode 117 of the controlled rectier 113 through a Zener diode 11-Sand a diode 119. A- resistor 1f20 and a capacitorlZ'l are connected in parallel between the gate electrode 117 land the junctionof the controlled rectifier and the-reset switch 11:4. A diode 1:22 anda 1,000 ohm resistor 123 are connected in parallel with the coil 115 of .the relay `116 and they contacts 124 are connected to the alarm unit 29.

The alarm unit 29 .includes a re alarm relay 125, a gong 126, a lamp 127, and .a ,gong cut-ott switch 1.29. The gong cuit-oil 'switch 129 has a pair of normally closed contacts and a pair ofA normally open contacts 131. The relay 12S .has a coil 132 and two pairs of normally open contacts 134, 135.. The coil 132 is connected in yseries with the contacts 130 of the Aswitch 129,l and-the contacts 124 of the relay 116 across the 30 volt D.C.

ysource. The lamp 127 is connected acrossthe 115 volt lA.C. source, both in series with the contacts V131 of the fswitch 129 Iand in series with the contacts 134 of the re- `ley 125.

The pulse generator control circuit 30 includes a transistor 136, a 220,000 'ohm resistor 137 connected'between the collector of the transistor 136 and the junction of resistor v64 land capacitor r65 in the pulse generator 24, a diode 139 connected between the em-itter of transistor 136 and the conductor 59, a 220 ohm resistor 140 connected between the base of the `transistor 136 andthe conductor 59, two 1,500 ohm resistors 141 and 142 connected in series between the transistor base and an input conductor 144, and a diode 145 connected between a second input conductor 146 and the junction of the resistors 141 and 142.

In the 'al-arm hold-off -circuit 31, the switch circuit 32 includes a transistor 147 havingits emitter connected to the positive conductor 106, a controlled silicon rectifier 148 having its positive terminal connected to the collector of the transistor'147 and having a gateelectrode 149, a capacitor 150 connected between the gate electrode 149 and the negative terminal ofthe controlled rectifier 148, 'a capacitor 151 connected betweenthebaseof the transistor 147 and the output of the pulse generator 24, and a 1,000 johm resistor 152 connected'between -the base of the transistor 147 and the negative conductor 107.

The delayed timing pulse generator 34 includes a unijunction transistor 153 having .a base 1 electrode 154 connected to the gate electrode 149 of the rectifier 148, a base 2 electrode 155 connected t-o the collector of transistor 147 through a 300 ohmresistor 156, and Ian emitter 157' connected to the collector of the transistor 147 through -a 500,000 ohm variable resistor'and a 150,000 ohm resistor 160 in series therewith and a diode 161 lin parallel with the resistors 159 and 160. 'Ihe Vemitter 157 and the base 1 electrode 154 are respectivelyY connected to the negative terminal of the controlled rectitier 148 through a V microfarad capacitor 162 and a 100-ohm resistor 164. Y

The smoke detector shunting circuit 35 includes a 100 ohm resistor 165 connecting the negative terminal ofthe controlled rectifier 148'to the negative conductor 107, a diode 166 connected betweern'the'I input of the amplifier 26 and the negative terminalof Vthe controlled rectifier `148, and a '.0005 niicrofarad capacitor '167 connected between thefinput of the amplier 26 and the .negative conductor 107. Y s i ,The pulse former 37 includes two 5,100 ohm resistors 169 and 170 connected inseries withv a 2 microfarad capacitor 171 and a 47,000 ohm resistor 172V between the negativeconductor 174 and the point Yin the'b-rake driver 25, a Vdiode 175 connected between a positive conductor 176 and the junction ofVV the resistors 169 and 170,and a diode-177 extending Yfrom the junction of the capacitor 171 and the resistor172 to the motion supervisor circuit 39. Y

The motion supervisor Vcircuit 39.includes two tran.`

sistors 179 and 180, a unijunction transistor 181, and a controlled silicon rectifier 182. A 160y ohm'resistor'184 4and 910 ohm resistor 185 are connected in series with the motion sens-ing unit 36 between the positive conductor 176 and the negative conductor 174. A Zener diode 186 is connected between Vthe negative conductor `17 4 and one side of the unit 36, and 4a 6 microarad capacitorV 187 is connected betweenthe base of transistor 179 and the other side of the unit 36. A 10,000 ohm resistor V189 and a diode 190 are connected in parallel between the negative conductor 174 and the base of the transistor 179.

The emitter of trans-istor 1'79 is connected directly to the base of transistor 180, a 10,000 ohm resistor 191 is connected between .the base of transistor 180 and the negative conductor 174, and the diode r177 of thepulse former 37 is connected to the base of transistor 180.

`The collectors of the transistors 179 and 180 are interconnected and are connected to the positive conductor 176 through a 100,000 ohm resistor 192 and 500,000 ohm variable` resistor 194.

Y The emitter of the unijunction transistor 1-81 is connected directly to the collector of `transistor 180 and is connected to the negative conductor 174 through an 18 microfarad capacitor 195 and a 10 microfarad capacitor 196 connected in parallel. The base 2 electrode is connected to the positive conductor 176 through a 300 ohm resistor 197 and the base 1 electrode is connected to the negative conductor 174 through a 100 ohm resistor 199. The controlled rectierv 182 is connected in series with a 2,700 ohm resistor 200 between the conductors 176 and 174,Y and the gate electrode thereof is connected to the positive end of resistor 199. i'

The fault indicator 40 includes a relay 201 having a coil 202 Vconnected across the resistor 200 and having a pair of contacts 204, a relay 205 having a coil 206 con- -lamp 209 connected in series with the contacts 207 across the volt A.C. supply. Y f

The fault signal prevent circuit 41 includes a transistor 210 having its `collector connected to the emitter of the unijunction transistor 181, a transformer 211 having its prim-ary winding connected across the lire indicating lamp 127, a diode 212 and a 1,300 ohm resistor 214 connected in series with the secondary winding of the transformer 211 between the negative conductor 174 and ther base of the transistor 210, a 510 ohm resistor 215 also connected between the conductor 174 and the base of transistor 210, and a diode 216 connected between the emitter of transistor 210 yand the conductor 174.

The automatic test switch 42 is connected in series with the ymanual test switch 45 and the coil 217 of a test relay 219 across ther 30 volt D.C. supply, and is also connected in serieswith the smoke simulator 46 across the 30 volt D.C. supply.V In addition, the test switch, 42 is connected in series with a set of normally open contacts 220 in the stopswitch 49 between theV input conductor 146 of the control circuit 30 land the emitter of the unijunction transistor'181 in the vmotion supervisor circuit 39.

The test relay 219 is provided with a pair of contacts 221 connected in series with the contacts of fire alarm relay 125, a pair of contacts 222 connected in series with .hold-on circuit 47, a stationary contact 225 connected to the positive side of electromagnet 22 through a diode 226, and a movable contact 227 connected to the positive side of the electromagnet 21through a diode 229.

The alarm hold-on circuit 47 includes a transistor 230 havingrits collector connected to the positive side of the relay coil 115, a diode 23.1 connecting the emitter of the transistor 230 to the relay contact 224, a diode 232 connecting the emitter of transistor 230 to the positive conductor 106, a`10,000 ohm resistor 234 connected between the'positive conductor106 and the base of the transistor 230, a 10,000 ohm resistor 235 connected between the negative terminal of the controlled rectifier 143 and the base of the transistor 230.

The manual stop` switch 49 has a second set of contacts l including a stationary contact 236 connected to the positive side of the relay coil 115, a second stationary contact 237 connected to the positive side of the 30 volt D.C. supply, and a movable contact 239. connected to the input conductor 144 of the control circuit 30.

The integrity supervisor circuit 48 includes a 900 ohm resistor 240 and a 500 ohm resistor 241 connected across the 115 volt A.C. supply; four diodes 242 to 245 arranged in a rectifying bridge connected between the junction of the resistors 240, 241 and junction of the contacts v135, 221; and a relay 246 having a normally ener- 9 gized coil 247 connected across the diode bridge 48, and a pair of normally open contacts 249 connected in series with the coil 206 of the fault relay 205 across the 115 volt A.C. supply. Such anintegrity supervisor circuit is disclosed and claimed in co-pending application Serial No. 212,097, filed July 24, 1962.

DETAILED OPERATION OF THE CIRCUTS The timing pulse generator 24 In operation, when the system is initially placed in operation, the transistor 84 in the brake driver 25 is in the non-conductng condition. As the voltage on the conductor 55 builds to +30 volts, current flows through the resistor 85, the diode 89, the resistor 911, and the resistor 80, and the electromagnet 21 to trigger the gate 75 of the controlled rectifier 70. The controlled rectifier '70 is thus turned on to provide a strong current flow through the electromagnet 21, and current also flows from the :point Y through the diode 94 and the resistor 86 to turn on the transistor 84 thus placing the positive terminal of the diode 89 at a potential near that of the negative conductor 59 to render the diode 89 non-conductive.

The negative terminal of the controlled rectifier 74) is now at a high positive potential while the negative terminal of the controlled rectlier 69 is at ground potential, and the capacitors 81 and 82 charge to this potential difference.

The controlled rectifier 70 remains turned on for 5 seconds during which time the capacitors 65 and 66 in the timing pulse generator 24 charge to the voltage required to fire the unijunction transistor 52. At the end of the five second period, the unijunction 52 hres to discharge the capacitors 65 and 66 and produce a voltage pulse across the resistor 57. This voltage pulse is differentiated by the capacitor 71 and the resistor 72 and is applied to the gate electrodes '74 and 75 of the controlled rectifiers 69 and 70 through the diodes 76 and 77.

This applied pulse turns on the controlled rectifier 69 and has no effect on the controlled rectifier 70 which is already conducting. When the rectifier 69 conducts, its negative terminal moves quickly toward the potential of the positive conductor 55, therefore, the charge on the capacitors 81, 82 raises the negative terminal o-f the controlled rectier 70 to a voltage more positive than the conductor 55 and the controlled rectifier 70 turns oli. Now current ows through the controlled rectifier 69 to the electromagnet 22. The diode 92 provides a continuous current through the resistor 89 to maintain the transistor S4 in conduction.

During the second period that the controlled rectifier 69 is conducting, the capacitors 81, 82 charge in the opposite 'direction so that the electromagnet 21 will become energized again in response to the next output pulse of the pulse generator 24.

A'Iarm'holdpy, ampliyer, and alarm energizing circuits; 31, 26, 27

When the system is initially placed in operation, the transistor 147 in the switching circuit 32 goes into conduction and the capacitor 162 begins char-ging through the emitter-collector circuit of the transistor 147 and the resistors 159 and 161i. After 3 seconds, the capacitor 162 achieves a charge which causes the unijunction 153 to fire producing an output pulse which is impressed 'on the control electrode 149 and turns on the controlled rectilier 148. Current then flows from the positive conductor 196 through the emitter collector circuit of the transistor 147, the controlled rectifier 143, and the resistor 165 in the shunting circuit 35 to the negative conductor 107. Dur-ing the period that the controlled rectifier 148 is conducting, the voltage across the delayed pulse generator 34 is held at nearly zero rendering the circuit inactive. When the first pulse is received from the pulse generator 24, the transistor 147 is cut off for the duration of the pulse and the controlled rectifier 148 10 turns 'off and the unijunction transistor 1'53 is reset. At the en'd of the pulse from the generator 24, the transistor 147 turns on again and the capacitor 162 charges for 3 seconds and fires the unijunction transistor 153 to turn on the rectifier 143 for the remaining 2 seconds before the next pulse from the generator 24 is received.

The photocell 96 in the detect-ing unit 10 is normally shielded from all light and has a very high resistance. When smoke is introduced into the unit 10 .the light from the lamp is reliected from the smoke and falls on the photocell causing it to drop in resistance to some lower value. Current lthen fiows from the volt supply through the photocell 96 and the resistor 97. During the first 3 seconds after each output pulse of the pulse generator 24, the controlled rectifier is not conducting therefore the negative side -of the diode 166 is at ground potential and any current flowing through the resistor 97 is shunted to the conductor 107 through the diode 166 and the resistor 165. During the next 2 seconds, the rectifier 148 is conducting and the negative side of the diode 166 is at a high positive potential, therefore, the diode 166 is reverse biased and any current flowing through the resistor 97 produces -a current flow in the base emitter circuit of the transistor 99 in the amplifier 26. This current flow is amplified by the transistors 99, 19t), and 161 to provide sufiicient power to turn on the controlled rectifier 113. The output of the transistor 1111 is fed through the zenerdiode 11S and the diode 119 to the gate electrode 177 of the controlled rectifier 113. When the controlled rectifier 113 is turned on, current flows from the positive conductor 106 through Vthe resistors 234 and 235, the rectifier 113, the reset switch 114, and the coil 115 of the relay 116 to the negative conductor 107. The coil 115 Yis thus energized and the contacts 1.24 are closed.

The Zener diode 118 serves as a voltage reference to prevent unwanted low level signals from triggering the controlled rectifier 113 to give a false alarm. The diode 119 protects the controlled rectifier V1.13 against reverse voltages which may injure the rectifier 113 when it is turned on.

Alarm unit 29 When the contacts 124 close, current is permitted to ow from the 30 volt D.C. source through the coil 132 of the relay 125, and the contacts 130 of the gong cut-'ofi switch 129, energizing the relay 125 to close the contacts 134 and 135. The Vclosing of the contacts 134 and 135 permits current to fiow from the 115 volt A.C. source through the lamp 127 and the gong 126. if it is desired to silence the gong 126, the switch 129 is thrown, opening the contacts 131) to de-ener-gize the relay 125, and closing the contacts 131 to connect the lamp 127 across the 115 volt A.C. source.

At the time the relay 116 is energized, a portion of the current through the controlled rectifier 113 flows through the contacts 236 and 239 of the stop switch 229 to the conductor 144, and through the resistors 142, 141 and 146 of the control circuit 31H0 turn on the transistor 136. The conducting transistor 136 then short circuits the capacitor 65 to render the pulse generator 24 inoperative so that the brake driver 25 holds the Vselector valve stationary with the detector 1'0 connected to the line 13 containing smoke.

After an alarm is given, the system is reset by operating the reset switch 114 to break the circuit through the controlled rectifier 113. The rectifier 113 is thereby turned off and the pulse generator 24 is re-activated to start the selector valve once again. If, at the time the reset switch 114 is operated, smoke is still present in the line 13 at which the selector valve was stopped, it is necessary to hold Vthe reset switch open until the selector valve moves to the next line.

Motion supervisor 39 In the motion supervisor circuit 39, the unijunction ergized.

`transistor 181 is controlled by the vcharge on the capaci- .tors 195 and 196. These capacitors charge through the l'resistors 192 and 194 and require about 8 seconds to charge sufficiently to turn the unijunction transistor 181 Vbase-ot' the Vtransistor 180. The transistor 180 is thereby turned on momentarily to again discharge the capacitors'195 and 196. The alternate pulses from the unit .36 and the pulse former 37 discharge the capacitors 195 and 196 every 5 seconds when the selector valve is operating correctly whereby the unijunction transistor 181 is prevented from firing. Should the valve stop rotating due to some malfunction, the pulses from the motion sensing unit cease and there is sutiicient time between the pulses from the pulse Vformer 37 to allow the capacitors 195 and 196 to charge to .the value required to cause the unijunction transistor 181 to tire. When the unijunction transistor 181 fires, it developsa positive pulse across resistor 99 `of sufficient magnitude to trigger the gate electrode of controlled rectifier 182. The rectier 182 turns on to complete a current path through the winding 206 of the fault relay 205. The contacts 207 of the relay 205 close placing the lamp 209 across t-he 115 volt A.C. supply.

Fault signal preventing circtiit 41 (FIG. 4)

Automatic test switch 42 Once during each revolution of the selector valve, the automatic test switch 42 is opera-ted. The closing of the switch 42 connects the smoke simulator 46 -across the 30 volt D.C. supply to direct light lonto the photocell 96, connect-s the coil 217 of the relay 219 across the 30 volt 'D.'C. supply, and connects the input conductor 146 of the control circuit 30 toiground potential to render ineffective the stop motion signal produced when the relay 116 is en- Alarmhold-n circuit 47 The operation of therelay 219 connects the emitter of the transistor 230 in the alarm hold on circuit 47 to the point Y in the brake driver 25 through the diode 231, the contacts 2.24 and 227 ,and the diode 229. The test switch operator 44 is positioned so that the test switch' is closed when the controlled rectifier '70V .is conducting to energize @the electromagnet 21. The transistor 230 therefore conductsand immediately energizes the relay coil 11-5 to operate the relay 125, however the gong 126 and the lamp 127 Vwill not operate because the relay contacts 221 and 222 `are now open. When, at the end of the 3 second Ihold-off period,` the controlled rectifier 148 is turned on,

the base of the transistor 230 is driven positive turning the transistor oli. If the system is operating properly, the output of the amplifier 26 will maintain the relay 125 energized with the contacts 13 4 and -135 closed. If the system is not working properly the integrity supervisor circuit will Operate as described below to energize the fault relay 205 and light the lamp 209.

Integrity supervisor circuit 48 In the integrity supervisor circuit 48, the coil 247 remains energized holding the contacts 249 open as long as a connection exists between either side of the 1'15 volt 12 A.C. supply and the junction of the dio-des 243 and 245. When the selecto-r valve is in the test position and the detecting unit 10, amplifier 26, and the alarm energizing circuit 27 are operated properly the -contacts 135 ofthe relay 125 are closed and current can l'low through the resistor 241, the diode 244, the coil 247, the diode 243 and the gong 126. This current fiow is sutiicient to energize the coil 247 and .hold the contacts 249 open but is not `suf- -licient to operate the gong 126. It will be seen that should the relay 125 fail to operate or should an open circui-t exist in the gong 126 the current fiow through the coil 247 will stop and the contacts 249 will close to energize the relay i 205 and light the fault lamp 209.

Manual test switch 45 lf at any time it is desired to test the operation of the gong 126 and the lamp 127, the manual test Yswitch 45 is operated. This switch produces thesame results as the automatic switch 42 with the exception Vthat the relay 219 is not energized and therefore the gong 126 and the lamp 127 are energized when the relay 125 operates.

Manual stop switch 49 Y The manual strop switch 49 can be operated at any time to stop the movement of the selector valve. The operation of the switch 49 closes contacts 239 and 237 to produce a current iiow through resistors 1142, 141, and to turn on the transistor 136 and short circuit the capacitor-65 to stop the operation of the pulse generator 24.

Fast step .switch 67 If at any time it is desired to rapidly move the selector valve to :any given position, the fast step switch 67 is closed to short out the resistor 61 thus increasing the rate at which the capacitor 65 is .charged `and decreasing the time interval between output pulses of the pulse -generator 24. With the switch 67 closed, the pulse generator produces a pulse every two seconds whereby the selector valve is rapidly stepped through its positions and the holdofi circuit remains energized until after the switch I67 is released so that the detect-ing unit 10 will not produce a response during this period. Y

From the foregoing, it will be seen that the present invention provides an improved electrical control system for lsmolie detecting apparatus which automatically operates, and monitors and periodically tests the condition of the apparatus and prevents the apparatus from Vgiving an erroneous indication. Y 4

Although the presentinventi-on is described herein in connection with the ysmoke detectingyapparatus disclosed in the aforementioned co-pending application, it is to be understood that this invention is not limited to this use and can be employed in connection withother detecting apparatus.

As various changes may be made in the form, construction and arrangement of the parts herein, without departing from the `spirit and scope of the invention land without sacrificing any vot its advantages, it is to be understood that all matter therein is to be interpreted as illustrative and not in any limiting sense.

We clairn:

1. In an electrical control system for condition detecting apparatus including a plurality of lines for transmitting the condition from spaces in which the presence of the condition is to be detected, a detect-or for giving an indication in Vresponse to the presence of the condition, .a rotary selecting device for connecting the detector to each of the lines, a motor for driving said rotary selecting device, :and brake means for stopping `said selecting device at predetermined positions to sequentially connect the detector -to each of the lines, said brake means including circumferentially spa-ced armature means mounted lon said selecting device for rotation therewith and a pair `of stationary electromagnets Vpositioned to cooperate with said armatures and being spaced differently lthan the armatures so that only one electromagnet is aligned with -an armature at a time; the lcombination. of timing means for producing electrical impulses at apredetermined interval, and a brake driving circuit having two outputs for producing on said outputs alternate square Wave pulses each having a duration equal to said predetermined interval, one of said outputs being connected to energize one of said electromagnets for one of said predetermined intervals of time and the other of said outputs being connecte-d to e-nergize the other ot said electromagnets for the next said predetermined interval of time so that each of said armatures co-operates first with one of said electromagnets and subsequently with the other of said electromagnets to stop the selectin-g device in -a number `of positions greater than the 4number of armatures.

2. In lan electrical control system for condition detecting 'apparatus including a plurality of lines for transmitting the condition from spa-ces in which the presence of the condition is -to be detected, a detector for giving an indication in response to the presence of the condition, a rotary selecting device for connecting 'the detector to each of the lines, a motor for driving said selectin:7 device, `and brake means for stopping said selecting device at predetermined positions to sequentially connect the detector to each of the lines, said ybra-ke means including circumierentially spaced rsteleme'nts mounted on said selecting device for rotation therewith and stationary second elements positioned to cooperate with said armatures to stop said selecting device; the combination of timing means for ycontrolling said motor means to connect each of said lines to said kdetector for a predetermined interval of time, and means for monitoring the operation of said moto-r means to give an indication when said motor means ceases to operate including a pulse producing device mounted adjacent said selecting device for producing an electrical pulse in response to the passing of each of said first elements, and means connected to said pulse producing device for giving an indication when the time interval between pulses exceeds a predetermined value.

3. Apparatus according to claim 2 including means responsive to the output vof said detector rfor stopping `the op eration 'of said timing means when said detector responds to the presence of the condition, and means responsive t-o the output' of said vdetector for `inerting said monitoring means when said detect-or responds to the vpresence of the condition.

4. In an electrical control system for conditioning detecting apparatus including 'a plurality of lines tor transmitting the condition from spaces in which the condition -is to be detected, a detector for giving an indication in response to the presence ofthe condition, a selector device for connecting the detector to each of the lines, and motor means for driving said device t-o sequentially connect the detector to each of the lines; the combination of timing means for controlling said motor means to connect each of said lines to said detector for 'a predetermined interval of time, means responsive to the `output of said detector for stopping the operation of said timing means when said detector responds to the presence of the condition, means connected to said detector for simulating the presence or the condition to test the operation of said detector, and means for rendering said stopping means ineffective when said condition simulating means is energized.

5. In an electrical control system for smoke detecting apparatus including a plurality of conduits for supplying air samples from spaces in which smoke is to be detected, a detector for giving an indication in resp-onse to the presence of smoke, a selector valve for connecting the detector to each of the conduits, and motor means for driving said valve to sequentially connect the detector to each of the conduits; the combination of timing means for controlling said motor means to connect each of said lines to said detector for a predetermined interval of time, and indication hold-olf means under the control of said timing means for rendering said detector ineiective during a predetermined initial .portion of said interval of time whereby said detector is prevented' from giving Van indication after said detector is `disconnectetl fromv a conduit transmitting smoke. r

`6. Apparatus according to claim 5 including means for simulating the presence of smoke at said detector for testing lthe operation of said detector, and means for rendering said indication hold-off means ineffective when said 'smoke Isimulating means is actuated. l

7. Apparatus according to claim 5 including means responsive Ito the output of said detector for stopping the operation of said timing means when said Vdetector responds to the presence of smoke. p

8. Apparatus according to claim 7 including means connected to said detector for vsimulating the prese-nce of smoke to test the operation of said detector, and means for rendering said stopping means ineffective when said smoke simulating means is energized.

9. Apparatus according to claim 8 including means for rendering said indication hold-olf mean-s ineie-ctive when `said'srnoke simulating means is actuated.

lu. In an electrical control system for 'smoke detecting apparatus including-a plurality of conduits for rsupplying air samples from spaces in wh'i-ch smoke 'is to be detected, a detector for giving an indication in response to the presence of smoke, a rotary select-or valve having a plurality of positions for connecting the detector to each of the conduits, and motor Ameans formo-ving said valve in lsteps through its various positions; the combination of motion detecting means associated with said motor means for producing an output pulse in 'response to the motion of 'said selector vvalve between vpositions thereof, an indicator, means for actuating said indicator, and time delay mean-s actuated by the output 'pulse of said motion detecting means Afor preventing the 'actuation of said indicator for a predetermined time after the occurrence `of each said output pulse. y

1l. lIn an electrical control system for smoke detecting apparatus including a plurality of conduitspfor supplying air samples from spaces in which smoke is -to be detected, a detector for givingan indication '.inresponse to the presence of smoke, a rotary selector valve for connecting the detector to each of the conduits, motor means for placing a constant torque on 'said selector valve, a dis-c mounted for rotation with said selector valves having a plurality lof armatures thereon, a braking member adapted to be energized to hold the armatures in predetermined rotational positions, and a second braking Imember adapted to be energized to 'hold the armatures in positions intermediate said predetermined positions; the cornbination of timing means for producing electrical impulses at -a predetermined interval, brake driving means having one output for energizing said rst Ibraking member for one of said predetermined intervals of time and having another output for energizing 'said second braking member for the next said predetermined interval of time so that said valve is moved in step-s and each of said lines is connected to said detector for said predetermined interval, motion detecting means associated with said stepping motor means for producing an output pulse in response to the passing of one of said armatures, an indicator, means for activating said indicator, 'and time delay means for preventing the actuation of said indicator when the selector valve moves in steps at said predetermined interval, said time delay means being connected to receive pulses alternately from said m-otion detecting means and one of said brake driver outputs.

12. In an electrical control system for smoke detecting apparatus includin-g a plurality of conduits for supplying air sa-mples from spaces in which smoke is to be detected, a detector for giving `an indication in resp-onse to the presence of smoke, a selector valve for connecting the detector to each of the conduits, and motor means for driving said valve to sequentially connect the dete-ctor to each ofthe conduits; the combination of timing means for vcontrolling said motor means to connect each of ysaid conduits to said detector for a predetermined interval of time, means for monitoring the operation of said motor means to give an indication when Isaid mot-or means ceases to operate, means responsive to the output-of said detector for stopping the operati-on of Isaid timing means when said detector responds to the presence of smoke, means responsive to the output of said detector for inerting said monitoring means when said detector responds to the presence of smoke, indication hold-off means for rendering said detector inetectiveduring a predetermined initial portion of said interval of time whereby any delay in the response of said detector is prevented from causing said detector -to give an indication after said detector is disconnected Yfrom a tube transmitting smoke, means connected to said 'detector' for simulating the presen-ce of smoke to test the operation of said detector, means for rendering said stopping means ineffective when said smoke Simulating means is energized, and means for rendering said indication hold-ott means ineffective when said smoke simulating means is actuated.

13. In an electrical control system for smoke detecting apparatus including a plurality of conduits for supplying air samples from spaces in which smoke is to be detected, a detector rfor givingjan indication in response to -the presence of smoke, a selector valve for connecting the Adetector .to each of the conduits, and motor means having a plurality, of electrically operableV devices Iadapted to be alternately energized to move said valve in steps to sequentially connect the detector to each of the conduits; the

combination of timing lmeans for producing electrical impulses at .a predetermined interval, driving means tor enengizing at least one of said devices tor one of said intervals and at least one other of said devices rfor the next interval so that each of said conduits is connected to said detector for said predetermined interval of time, means for monitoring the operation of said motor means to ygive an V4indication when said motor means ceases to operate, means responsive Ito the output of said detector rfor stopping the operation of said timing means when said.v

detector responds to the presence of smoke, means'responsive to the output of said dete-ctor for inerting said monitoring means when saidv detector responds to the presence ot smoke, indica-tion hold-oit means .for rendering said detector ineiective during a predeterminedV initial portion of said interval of time whereby any delay in the response of said detector is prevented strom causing said detector to give an indication after said detector is disconnected yfrom a tube transmitting smoke, means connected to said detector ffor Vsimulating the presence of smoke to test the operation of said detector, means for rendering said stopping means ineffective when said smoke simulating means is energized, and means for rendering said indication hold-off means ineffective when said smoke simulating means is actuated.

14. In an electrical control system for smoke detecting apparatus including a plurality of conduits for supplying air samples from spaces in which smoke is to -be detected, a detector for giving an indication in response to the presence of smoke, a rotary selector valve -for connecting the detector -to each of the conduits, and a plurality ot elec- .trically operable motor means adapted to Ibe alternately enengized to move said valve in steps to sequentially connect said detector to each of said conduits; the combination of timing means for producing electrical impulses at a predetermined interval, motor means driving means having an input connected to said timing means and havig a pair of outputs connected to d-ifrerent motor means, said driving means being constructed and arranged to provide an output signal to one or the other of saidY outputs at all -times and to transfer the signal between said outputs in response .to each input impulse, and means responsive to the output ot said detector for interrupting vthe operation of said timing means whereby one of said outputs of said driver means is maintained in an energized condition to interrupt the movement ofsaid selector valve.

15. Apparatus according to claim 14 including indication hold-off means under the control of said timing means -for rendering said detector ineffective during a predetermined initial portion ot said interval of time whereby said detector is prevented from lgiving an indication after said detector is disconnected trom a conduit transmitting smoke.

References Cited by the Examiner UNITED STATES PATENTS 2,034,281 Y 377/1936 Buchholz 340-237 2,639,418 5/1953 Sundstrom et a1. 340-214 2,762,014 9/1956 Anderson 340-213 2,763,853 9/1956 Grant 340-214 3,048,820 8/1962 Derr et al 340-163 3,107,340 10/ 1963 Silliman et al 340-213 NEIL C. READ, Primary Examiner.. 

5. IN AN ELECTRICAL CONTROL SYSTEM FOR SMOKE DETECTING APPARATUS INCLUDING A PLURALTIY OF CONDUITS FOR SUPPLYING AIR SAMPLES FROM SPACES IN WHICH SMOKE IS TO BE DETECTED, A DETECTOR FOR GIVING AN INDICATION IN RESPONSE TO THE PRESENCE OF SMOKE, A SELECTOR VALVE FOR CONNECTING THE DETECTOR TO EACH OF THE CONDUITS, AND MOTOR MEANS FOR DRIVING SAID VALVE TO SEQUENTIALLY CONNECT THE DETECTOR TO EACH OF THE CONDUIT; THE COMBINATION OF TIMING MEANS FOR CONTROLLING SAID MOTOR MEANS TO CONNECT EACH OF SAID LINES TO SAID DETECTOR FOR A PREDETERMINED INTERVAL OF TIME, AND INDICATION HOLD-OFF MEANS UNDER THE CONTROL OF SAID TIMING MEANS FOR RENDERING SAID DETECTOR INEFFECTIVE DURING A PREDETERMINED INITIAL PORTION OF SAID INTERVAL OF TIME WHEREBY SAID DETECTOR IS PREVENTED FROM GIVING AN INDICATION AFTER SAID DETECTOR IS DISCONNECTED FROM A CONDUIT TRANSMITTING SMOKE. 